A tip end portion 7 of a discharge chute is constituted by inverted truncated cone-shaped internal and external chutes 23, 24, a space S is provided between an outer surface of the internal chute 23 and an inner surface of the external chute 24, and a first air discharge port 26 that communicates with the space S is provided between respective lower ends of the two chutes. A cylindrical portion 27 having an air reservoir 27' in the interior thereof is provided to surround an outer peripheral surface of the external chute 24, and by providing an opening between an inner peripheral edge of a lower surface of the cylindrical portion 27 and the outer peripheral surface of the external chute 24, a second air discharge port 28 that communicates with the air reservoir 27' is provided. Air input pipes 25, 29 for introduction of dry air are connected to the external chute 24 and the cylindrical portion 27, respectively, and the dry air can be ejected through the first and second air discharge ports 26, 28.
